An existing case-control study designed to evaluate the role of drugs (other than those used for chemotherapy and immunotherapy) in the etiology of aplastic anemia, agranulocytosis, and thrombocytopenic purpura will be continued. Cases will be indentified by regular examination of admission lists, laboratory data, and other sources in hospitals in eastern Massachusetts and Rhode Island where blood dyscrasias are treated, or by contacting appropriate hospital personnel by telephone, The total population fo the region is about 5.7 million. the final eligibility of the cases will be determined by a committee of hematologists, according to strict diagnostic criteria. Cases and appropriate hospital controls will be interviewed to obtain information on drug use, occupational and chemical exposures, personal data, and relevant medica history. It is expected that over three and one half years, 30 cases of aplastic anamia, 25 of agranulocytosis, 120 of thrombocytopenic purpura, andd 1200 controls will be added to the existing data base. By the end of the study, there should be at least 70 cases of aplastic anemia, 65 of agranulocytosis, 310 of thrombocytopenic purpura, and 2900 controls available for analysis. The data will be used to quantify known associations between the three dyscrasias and drug use, to indentify and quantify previously unsuspected associations, and to document absence of associations for commonly used drugs. The overall incidence of aplastic anemia will be estimated directly; the incidence of agranulocytosis and thrombocytopenic purpura will be estimated in a separate survey of hospital records conducted as part of the study. Incidence rates of the dycrasias attributable to specific drugs and classes of drugs will also be estimated. Blood dyscrasias, although rare, are important and serious side effects of numerous drugs. Quantitative information on risk is generally lacking. Based on interim analyses, the additional data yielded by the extension will greatly increase the power of the study to provide such information. The last six months of the grant will be devoted to final analyses of all the data.